The present invention relates to an optical head for reproducing an information signal from an optical information-recording medium, or performing both a recording operation and a reproducing operation for the optical information-recording medium, and more particularly to an optical head in which a focusing error signal according to the astigmatism method and a tracking error signal according to the push-pull method are both detected by a single optical system, and the focusing error signal and the tracking error signal can be adjusted independently of each other.
FIG. 1 shows a conventional optical head for detecting a focusing error signal according to the astigmatism method and a tracking error signal according to the push-pull method.
Referring to FIG. 1, a light beam emitted from a semiconductor laser 1 is converted by a collimator lens 2 into a light beam formed of parallel rays. The light beam from the collimator lens 2 passes through a polarization beam splitter 3 and a quarter wavelength plate 4, and is then deflected by a reflecting mirror 13. The deflected light beam is focused by an objective lens 5 on a disk 21 which serves as an information recording medium. head small in size and weight.
Further, another optical head is proposed in a Japanese patent application JP-A-63-161,543. In this optical head, a detecting optical system is not divided by the beam splitter into the optical system for detecting the focusing error and the optical system for detecting the tracking error, but includes a diffraction grating made up of a plurality of regions which are different from each other in the direction of diffraction or the angle of diffraction. Thus, a light beam which is reflected from the disk and then reflected from the polarization beam splitter is diffracted by each region of the diffraction grating, and a focusing error signal and a tracking error signal are detected from diffracted light beams having the order of diffraction equal to +1 (namely, an order of diffraction of +1). Further, an information signal is detected from the diffracted light beam of the zero-th order. Thus, the focusing error signal, the tracking error signal and the information signal can be obtained by a single detecting optical system.
In the proposed optical head, however, the following problem arises because the focusing error signal and the tracking error signal are both detected from the diffracted light beams having an order of diffraction of +1. That is, when the oscillation wavelength of the semiconductor laser serving as a light source varies, the angle of diffraction at each region of
Light reflected from the disk 21 is incident on the polarization beam splitter 3 through the objective lens 5, the reflecting mirror 13 and the quarter wavelength plate 4, and is reflected from the polarization beam splitter 3. The light reflected from the polarization beam splitter 3 is incident on a beam splitter 22. The reflecting surface of the beam splitter 22 has a predetermined transmittance and reflectivity. Thus, the light incident on the beam splitter 22 is separated into transmitted light and reflected light.
The reflected light passes through a detection lens 6, and then passes through a cylindrical lens 7 whose cylindrical axis is inclined at an angle of about 45.degree. to a predetermined direction such that the reflected light is provided with astigmatism. The reflected light having passed through cylindrical lens 7 is incident on a photodetector 24, and the output thereof is used for detecting a focusing error according to the astigmatism method. Further, the transmitted light from the beam splitter 22 is incident on a photodetector 23, and the output thereof is used for detecting a tracking error according to the push-pull method.
In the above optical head, the positions of the photodetectors 23 and 24 can be adjusted independently of each other, but an optical system for detecting the focusing error and an optical system for detecting the tracking error are different systems. Accordingly, there arises a problem that it is difficult to make the optical the diffraction grating is also varied, and thus the position of a light spot on a photodetector varies. At this time there arises a problem that a correct, focusing error signal cannot be obtained if the light spot on the photodetector moves in a direction other than a specified direction (that is, the direction of a boundary between a pair of detecting regions on the photodetector). In order to solve this problem, it is necessary to locate the diffraction grating accurately in relation to the photodetector so that the light spot on the photodetector moves only in the specified direction.
Further, in order to obtain a correct tracking error signal, it is necessary to adjust the tracking error signal previously. In the Japanese patent application referred to above, however, no attention is paid to the previous adjustment of the tracking error signal.
Furthermore, in the proposed optical head, the diffracted light beams having an order of diffraction of +1 are used for detecting the focusing error signal and the tracking error signal, and the diffracted light beam of the zero-th order is used for detecting the information signal. The diffracted light beams having an order of diffraction of -1, however, are not used for detecting any signal. Accordingly, the proposed optical head is low in utilization efficiency of light emerging from the diffraction grating.